Bauhaus-Universität Weimar

Experimental Psychology: A Manual of Laboratory Practice, Vol. I: Qualitative Experiments, part 2: Instructor's Manual
Titchener, Edward B.
§ 46. The Mechanism of Accommodation 
De examine physiologico organi visus et systematis cutanei, 
1823. They were employed for diagnostic purposes by the 
French surgeon L. J. Sanson (1790-1841 ; Leçons sur les 
maladies des yeux, 1837), and are now generally known by his 
The essentials of the experiment are as follows. The subject 
sits in a dark room, from which all light but that of the experi¬ 
menter’s candle is excluded. One eye is closed by a bandage. 
A far and a near fixation-point are marked (say, at 30 and at 300 
cm.) along the line of vision of the open eye. In front of this 
eye, to one side and upon the same level, is placed the candle 
flame ; the experimenter, who is observing the eye, sits on the 
other side of the subject, in such a position that the angle ESf 
is approximately equal to the angle CSf. After a little shifting 
of the eyes to and fro about the point E, the experimenter will 
Fig. 45.—Helmholtz, Phys. Optik, 1896, 132. E, experimenter; S, subject; 
C, candle; n, near; / far. 
see three reflected images of the flame in the eye 5. The first 
(nearest the light) is very bright ; it is an erect, virtual image 
from the convex surface of the cornea. The middle image is 
that reflected from the less convex anterior surface of the lens : 
an erect, virtual image, larger than the corneal, and apparently 
situated some 8 to 12 mm. behind the centre of the pupil. This 
image is very indistinct (“more like a light cloud than an 
image,” as Sanford remarks : Lab. Course, 94), and owing to 
its position is easily lost with any shift of C or E. The third 
image, lying on the nearer edge of the pupil, at an apparent 
depth of i mm., is reflected from the concave posterior surface


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